The present invention relates to a method of stripping paint and ink from a substrate. The stripper comprises an aqueous phase and an organic phase.
With the introduction of new and more durable types of synthetic resins and polymers in protective and decorative coatings, the problem of stripping has become increasingly difficult. Prior art paint removers typically contain blends of aromatic solvents, ketones, paraffin, methanol and diluents, for example, mineral spirits in order to reduce raw material costs. Those prior art strippers made with the foregoing solvents focused industry attention on the flammable and toxic characteristics.
As an alternative, improved prior art solvents utilize chlorinated solvents as a way to extend the usefulness of paint strippers while simultaneously reducing fire hazards associated with their use. Methylene chloride (dichloromethane) has been almost universally used as the solvent of choice, although some compositions have been formulated with ethylene dichloride and trichloroethylene, among other chlorinated hydrocarbon solvents. Many of the chlorinated hydrocarbons including methylene chloride are suspected carcinogens and methylene chloride is designated a hazardous substance under the Federal Hazardous Substance Act (FHSA). In addition, methylene chloride, as a high vapor pressure chlorinated solvent, probably contributes to atmospheric ozone depletion. Methanol is a poison if ingested and the U.S. Consumer Products Safety Commission requires a warning designating it as a poison when its concentration in consumer products exceeds 4 percent by weight. Toluene contains trace quantities of benzene, a known carcinogen, and acetone with its high vapor pressure and low flash point is highly flammable.
Paint strippers which include methylene chloride can be formulated to soften, lift and blister paint films. The stripper which is applied to the substrate by brush, immersion or spray and allowed to loosen the paint or varnish has an available work life of only several minutes because of the tendency of the methylene chloride to evaporate rapidly. Because of the high vapor pressure of methylene chloride, in an effort to extend the work life of the stripper made with that solvent, paraffin is often included as an evaporation suppressant. The paraffin functions by forming a thin film at the surface of the paint remover which retards evaporation. This thin film formation and resultant evaporation suppressant activity of paraffin is produced by the evaporation and surface chilling which occur when methylene chloride or mixtures containing methylene chloride are exposed to air. However, methylene chloride strippers must be handled carefully since they are highly toxic and can cause severe skin irritation. Further, even with the incorporation of paraffin, the volatility of the methylene chloride is still very high and when methylene chloride is formulated with, for example, toluene, the resulting high levels of hydrocarbon emissions necessitate the provision of good ventilation to safely use the methylene chloride paint strippers.
A further disadvantage of paint strippers formulated with methylene chloride is that the short work life compromises the ability of the stripper to function efficiently in many applications. For example, many industrial uses of paint strippers involve stripping accumulated paint from equipment and walls. Paint strippers utilizing methylene chloride suffer from the disadvantage that the short work life prevents the stripper from entering crevices in the accumulated paint layers thus creating the necessity, in some cases, of applying a large number of applications of stripper to remove a number of accumulated paint layers.
U.S. Pat. No. 6,348,107, issued Feb. 19, 2002, discloses a new method of stripping paint from a painted substrate surface. According to the invention, the surface is contacted with the aqueous phase of a two-phase liquid stripper composition comprising an aqueous phase which comprises water, dissolved water-soluble activator compound, and an organic solvent which has a solubility in water in the range 0.1 to 10% by weight at 20° C., the organic solvent being present in the aqueous phase at a concentration of about the saturation level (as at the ambient conditions for the method) and comprising also an organic phase comprising the said organic solvent, which organic phase is in interfacial contact with the aqueous phase. The organic solvent may include chlorinated hydrocarbons, for instance in combination with other organic solvents.
Suitable organic solvents for use in U.S. Pat. No. 6,348,107 include benzyl alcohol, methylene chloride, dibasic ester, which is an oxygenated solvent composed of a mixture of methyldiesters of glutaric, succinic and adipic acids, or ethyl-3-ethoxy propionate. The dissolved activator compound present in the aqueous phase of the composition may be any compound which increases the paint stripper activity. Such compound may often be a pH modifying compound in the form of an acid. Acid activators are usually organic acids, for instance trichloroacetic, alkyl benzene sulphonic acid, benzoic or lactic acid. The pH modifying compound can also be an alkali, for instance, alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal silicates, and organic basic compounds such as amines or ammonium compounds. Another category of activator compound is an oxidizing agent. Examples of an oxidizing agent include hydrogen peroxide and potassium permanganate.
It is known to use water as an activator compound in organic solvent based paint strippers. Such compositions tend to have low levels of water, for instance less than 10%. Other paint stripper compositions comprising both water and organic solvents which are immiscible with water have been described. Generally emulsifiers are added in quantities such that an emulsion of one phase in the other phase is formed. Such emulsions are storage stable and are required not to separate before use. Alternatively, co-solvents may be added to create a homogeneous single phase system.
Single phase paint strippers containing ethylene carbonate or propylene carbonate and additional solvents such as esters, ethers, alcohols, or pyrrolidones such as N-methylpyrrolidone are known and exemplified by U.S. Pat. Nos. 5,288,335 and 5,449,474. Alkylene carbonate solvents, along with hydrogen peroxide, in an aqueous-based system are disclosed in a series of patents assigned to Huntsman Petrochemical Corporation. Examples of these are U.S. Pat. Nos. 6,040,284; 6,162,776; 6,369,009; and 6,608,012, among others.